1. Field of the Invention
The present invention relates to treatment of waste water containing highly toxic developers and surfactants used in great amounts in semiconductor factories, liquid-crystal factories, and other various types of industrial facilities and more particularly to a developer-containing waste water treatment method and apparatus having a plurality of biological treatment stages.
2. Description of the Prior Art
Generally, developer-containing waste water exhausted from semiconductor factories, liquid-crystal factories, and other various types of industrial facilities has a high toxicity when its primary component is tetramethyl ammonium hydroxide (hereinafter, referred to as "TMAH"). Furthermore, the waste water contains large amounts of surfactants. For these reasons, it is difficult to biologically treat such waste water with microorganisms.
Conventionally, in order to give such waste water a biological treatment at a factory from which the waste water is produced, it is diluted until its concentration gets low enough for the biological treatment. Otherwise, the waste water is transported to an incinerating equipment located at a different place, where it is incinerated. The incineration is a reliable disposal method of waste water, but involves the transportation cost to the incineration facilities, a danger of traffic accidents during the transportation, and the incineration cost.
Furthermore, as the semiconductor factories and liquid-crystal factories become larger in scale and produce larger amounts of developer-containing waste water each day, the incineration of waste water causes the following problems. First, the construction cost of incineration facilities increases. Second, the cost of transportation of the waste water to the incineration facilities located at a different place increases. Third, when heavy oil is used as a fuel for the incineration, the fuel cost increases. In addition, the costs of measures to be taken against air pollution also increase. The maintenance of the incineration facilities also becomes more expensive. Besides, the incineration treatment itself involves waste of energy, which means that the incineration treatment is no longer a method suitable for these days when protection of the earth environment is called for.
FIG. 1 shows a prior art waste water treating apparatus which dilutes a TMAH-containing waste water and biologically treats the diluted waste water. In the waste water treating apparatus shown in FIG. 1, the raw developer-containing waste water first flows into an adjustment tank 1 before being treated. This is because water quality and quantity of waste water flowing into the apparatus are different at different times and it is not advisable to introduce such variable raw waste water directly into a tank to treat it. The adjustment tank 1 has an air diffuser pipe 10 installed therein. The raw developer-containing waste water in the adjustment tank 1 is stirred by air emitted from an air diffuser pipe 10 so that different water qualities of waste water flowing at different times are made uniform in the adjustment tank. In other words, water quality is adjusted. Water quantity is also adjusted. The adjusted developer-containing waste water is transferred by a pump 7 to an adsorption column 2, where surfactants contained in the developer-containing waste water are mainly adsorbed on activated carbon. The developer-containing waste water treated in the adsorption column 2 is then introduced into a dilution tank 3, to which diluent water previously stored in a diluent water storage tank 13 is injected by a pump 12, whereby the developer-containing waste water is diluted. The dilution depends on the TMAH concentration of the waste water, but is generally set at 10 times or so in most factories. As a matter of course, waste water is diluted until TMAH contained therein becomes biologically treatable. The waste water sufficiently diluted in the dilution tank 3 is neutralized in a neutralization tank 4 having an air diffuser pipe 10 installed therein. Since the developer itself is alkaline, an acid is added for the neutralization of the developer-containing waste water even after dilution. The waste water in the neutralization tank 4 is agitated by air emitted from the air diffuser pipe 10.
In FIG. 1, numeral 5 denotes an aeration tank 5. TMAH contained in the developer-containing waste water is treated primarily in this aeration tank 5. The air set forth from the blower 8 and output from the air diffuser 10 keeps the interior of the aeration tank 5 aerobic. Within the aeration tank 5, aerobic microorganisms live and breed.
Further, in FIG. 1, numeral 135 denotes a urea storage tank, and numeral 137 denotes a phosphoric acid storage tank. The urea storage tank 135 and the phosphoric acid storage tank 137 have their respective pumps 136, 138. Urea is fed from the urea storage tank 135 to the aeration tank 5, while phosphoric acid is fed from the phosphoric acid storage tank 137 to the aeration tank 5.
As stated above, TMAH is toxic and difficult to biologically treat. Therefore, the volumetric load of TMAH per unit volume of the aeration tank 5 is restricted to a small value. Accordingly, the volume of the aeration tank 5 needs to be relatively large.
Since TMAH is to microorganisms as noted above, nutritive agents such as urea and phosphoric acid are supplied in greater amounts in the treatment of TMAH than in the treatment of harmless organic substances. Also, TMAH being a toxic substance, the volumetric load of TMAH applied to the aeration tank 5 per day needs to be controlled to approx. 1 kg or less for 1 m.sup.3 of the aeration tank. Generally, waste water treating equipment having an inflow load of 1 kg or less per m.sup.3 of the aeration tank is relatively large for the waste water amount to be treated.
In FIG. 1, a settling tank 6, separates sludge containing aerobic microorganisms from supernatant liquid. Adjacent to the settling tank 6 is a sludge return pump 9 which serves to return to the aeration tank 5 the sludge containing aerobic microorganisms that have settled in the settling tank 6.
In the waste water treatment apparatus utilizing microorganisms, shown in FIG. 1, since the TMAH concentrations of developer-containing waste water from semiconductor factories are generally as high as approx. 2000 ppm to 10000 ppm, the developer-containing waste water is diluted as much as 10 times or so on average. Thus, the amount of the diluted waste water becomes so large that the size of the aeration tank 5 and the settling tank 6 increases. As a result, the waste water treatment apparatus increases in scale and therefore initial costs increase.
The operational cost of the above prior art waste water treatment apparatus of FIG. 1 is also high for the following reasons. Because of the increased size of the aeration tank 5, the blower 8 is required to set forth a large amount of air especially to agitate the waste water in the aeration tank 5 as well as to maintain aerobic microorganisms therein. Thus, the operation of the blower 8 consumes considerable electric power. In addition, in order to maintain microorganisms in a good condition in the aeration tank 5 against the toxicity of TMAH, it is necessary to feed the microorganisms with nutritive agents such as urea and phosphoric acid.
In the biological treatment of the waste water, if the condition of microorganisms deteriorates, the waste water within the waste water treating facilities such as the aeration tank 5 or the settling tank 6 is consigned to a dealer as an emergency measure. In this case, because the water quantities in the aeration tank 5 and the settling tank 6 are so large, the disposal cost is high.
The developer-containing waste water contains not only TMAH but also great amounts of various surfactants. Although those surfactants, which are mainly alkyl ammonium-based or polyoxyethylene-based, are necessary chemical substances for a development process in the semiconductor and liquid-crystal manufacturing processes, they cause other new problems in the biological waste water treatment. For example, surfactants prevent aerobic microorganisms from decomposing TMAH, due to their germicidal action on the microorganisms. In addition, the surfactants cause water to foam. This is why in the prior art the adsorption treatment of surfactants using activated carbon is performed in the adsorption column 2 as a pretreatment of the waste water. However, since the surfactants in the developer-containing waste water are at high concentrations and the adsorption ability of the activated carbon generally is limited, it is necessary to regenerate the activated carbon in a short time cycle. For the regeneration, the activated carbon is taken out from the adsorption column. After regenerated, the activated carbon is restored to the adsorption column. The work related to the regeneration of the activated carbon involves considerable labor and regeneration costs. The running cost and maintenance cost of the waste water treating facilities accordingly increase.
It is expected that as the microfabrication progresses in semiconductor and liquid crystal apparatus, surfactants will be contained in various types of chemicals including developers for the purpose of performance enhancement in the development process, i.e. yield enhancement
In addition, governmental regulation of specific fluorocarbons has a great impact on the electronics industry. On this account, various types of cleaning agents are being discussed as the alternative to conventional fluorocarbon-based cleaning agents, and attention is given to cleaning agents containing a) ultra-pure water and surfactants, b) limonene and surfactants, and c) a silicone solvent and surfactants, taking into consideration cleaning power and the risk of damaging parts to be cleaned.
The amounts and kinds of surfactants to be used in the electronics industry tend to increase, whether the surfactants are used in the alternative cleaning agents or in chemicals such as developers. There is also the possibility that semiconductor and liquid crystal manufacturing processes require use of surfactants which are difficult to biologically treat or that the amounts of surfactants to be used increase to such a level that they are difficult to biologically treat.
Recently environmental protection regulation is growing stricter. Current requirements imposed on waste water treating facilities is that waste water, even when discharged directly into rivers or seas, should not affect creatures living there, such as fish. However, since TMAH is a nitrogen compound, a simple biological treatment of TMAH-containing waste water decomposes TMAH to ammonia nitrogen and nitrite nitrogen which exhibit toxicity to fish. As a result, the waste water biologically treated contains those toxic substances, so that fish cannot live in such water. In order for fish to live in treated water, all of the following three requirements should be satisfied.
1) the water should be neutral;
2) the ammonia nitrogen and nitrite nitrogen concentrations of the water should be 1 ppm or lower; and
3) nitrogen compounds derived from TMAH should have been completely oxidized into nitrate nitrogen.
However, with conventional biological waste water treatment methods such as an activated sludge method, it is impossible to treat waste water containing a developer and surfactant such that all of the above three requirements are satisfied. For example, waste water treated by the activated sludge method contains ammonia nitrogen and nitrite nitrogen at high concentrations, and has a poor transparency, and the SS (suspended solids) concentration is above 10 ppm. To the knowledge of the inventors, there are no waste water treating facilities that are efficient, economical and compact and that meet the foregoing requirements for the treated waste water enabling fish to live therein.
Large-scale semiconductor and liquid crystal factories have their own facilities for treating household waste water derived from dining rooms, baths, lavatories, etc. used by employees. The household waste water treating facilities produce excess sludge having good nutritive balance. Hitherto, however, there has been no particular way for making effective use of such excess sludge in the semiconductor and liquid crystal factories, with the excess sludge requiring costly disposal.
As shown above, rational treatment methods for wastes derived from factories such as semiconductor and liquid crystal factories are today's.